Olive Oil Assays May Help Assure Authenticity

When the label on a bottle of olive oil misrepresents what's inside, shoppers may not be getting what they thought they paid for. Mislabeling is also of concern to chefs, retailers, and America's olive growers and olive oil processors—especially those newly entering the domestic olive oil market. They face tough international competition: An estimated 98 percent of all olive oil consumed in the United States is imported.

Help may be on the way in the form of laboratory assays developed by U.S. Department of Agriculture (USDA) researchers and their colleagues. These tests add to the array of options for quality-control analyses of olive oil.

Agricultural Research Service (ARS) chemist Talwinder Kahlon's assay relies on PCR (polymerase chain reaction) technology to compare olive DNA to that of canola and sunflower plants. Oil from these plants is sometimes mixed with olive oil, but not disclosed on the label. ARS is the principal intramural scientific research agency of USDA.

Kahlon's test focuses on key regions of two genes, matK and psbA-trnH, which occur widely throughout nature, including in olive, canola, and sunflower. The DNA sequence of specific regions of these two genes, Kahlon notes, provides a reliable basis of comparison, and can be used to detect the presence of the non-olive oils at concentrations of 5 percent or higher.

Though using PCR technology to detect specific plant DNA in olive oil isn't new, the team's approach offers several improvements. For instance, the olive, canola, and sunflower "DNA barcodes" that the scientists developed—to serve as the basis for comparing these plants' DNA—are based on not just a single olive tree or sunflower or canola plant. Instead, each barcode is a broadly representative composite, known as "consensus DNA."

Olive oil is made up of triglycerides, which are molecules composed of fatty acids. These fatty acids are the focus of the approach that ARS chemist Jiann-Tsyh (Ken) Lin developed. The assay is based on ESI-MS (electrospray ionization-mass spectrometry) and enables scientists to glean details about variations in specific triglycerides of interest, referred to as regioisomers. From that, users can develop ratios of regioisomers that can be used to determine whether the sample contains undisclosed oils.

The value of ESI-MS for analyzing plant fatty acids has been recognized since at least 1994. But Lin's ESI-MS protocol helps make this application simpler.

Lin developed the protocol for his research with castor, a plant that produces an inedible, top-quality industrial oil. About six years ago, he chose olive oil as a model for testing his assay.